X Primary Endocrine Disease (Thyroid, adrenal cortex, gonads, liver) •Fig. 41.6 Negative-feedbackloopsregulatinghormonesecretioninatypicalhypothalamus-pituitaryperipheralglandaxis.X,peripheralglandhormone;XIH,hypothalamic-inhibitinghormone;XRH,hypothalamic-releasinghormone;XTH,pituitarytropichormone.
short-term (e.g., various stress/infections) or long-term (e.g., onset of reproductive function at puberty) inputs. Thus inclusion of the hypothalamus in an endocrine axis allows integration of a considerable amount of information for setting or changing the set point of that axis. Clinically this means a broad range of complex neurogenic states can alter pituitary function. Psychosocial dwarfism is a striking example in which children subject to abuse or intense emotional stress have lower growth rates as a result of decreased growth hormone secretion by the pituitary gland.
3. Abnormally low or high levels of a peripheral hormone (e.g., thyroid hormone) may be due to a defect at the level of the peripheral endocrine gland (e.g., thyroid), the pituitary gland, or the hypothalamus. Such lesions are referred to as primary, secondary, and tertiary endocrine disorders, respectively (see
Fig. 41.6 ). A thorough understanding of the feedback relationships within an axis allows the physician to determine where the defect lies. Primary endocrine deficiencies tend to be the most severe because they often involve complete absence of the peripheral hormone.
Endocrine Function of the Adenohypophysis
The adenohypophysis consists of the following endocrine cell types: corticotropes, thyrotropes, gonadotropes, somatotropes, and lactotropes (see
Table 41.1
Corticotropes stimulate the adrenal cortex as part of the hypothalamic-pituitary-adrenal (HPA) axis. Corticotropes produce the hormone ACTH (corticotropin), which stimulates two zones of the adrenal cortex (see ). ACTH is a 39–amino acid peptide that is synthesized as part of a larger prohormone called proopiomelanocortin (POMC). Thus corticotropes are also referred to as POMC cells. POMC harbors the peptide sequence for ACTH, two isoforms of melanocyte-stimulating hormone (MSH), endorphins (endogenous opioids), and enkephalins (
Fig.
41.8 ). However, the human corticotrope expresses only pro-hormone convertase-1, which produces ACTH as the sole active hormone secreted by these cells. The other fragments that are cleaved from POMC are the N-terminal fragment and β-lipotropic hormone (β-LPH), neither of which plays a physiological role in humans.